I was running around Diamond City the other day picking up scrap items and in my haste accidentally stole an "owned" item when all of a sudden everybody and their kid brother starting shooting at me, including my follower. I wasn't wearing any armor and as I was running away trying to pump myself full of stimpaks in order to stay alive, a screen popped up to reassign my follower to another location. I hit the cancel tab to stop the relocation process, or so I thought, then quickly ducked into Home Plate before I died. I slept an hour to restore my hit points and the game autosaved as it does. when I looked around, my follower was gone and now I cant find them anywhere. how do I find where they went?

They went back to where they originally were, so if it's Piper she'll be back in the publick occurrences.

They would have gone back to whatever settlement you last left them at, and if they were never assigned to a settlement they'd go back to their default location (Valentine Detective Agency for Nick, Publick Occurences for Piper, and so on).

I've had this happen before, and it's really frustrating. I just reloaded an earlier save though because I didn't want to lose the affinity points.